1. Field of the Invention
This invention relates to irrigation control systems, and more particularly to automated and semi-automated irrigation control systems for large or small landscape sites using evapotranspiration and precipitation data.
2. Description of Related Art
The term evapotranspiration (ET) is used in the irrigation field to quantify how much water has been lost from soil through transpiration by plants. An ET value is calculated using actual meteorological data obtained from meteorology stations. The factors typically used to calculate an ET value are temperature, solar radiation, wind speed, vapor pressure or humidity, and barometric pressure. A change in one or more of these parameters can have a direct effect on the ET value used to determine when and how much to water. ET values are usually normalized to a specific type of vegetation. One of these ET values is ETo which is for 4"-6" tall cool-season grass. ET values are then used in conjunction with other coefficients to determine how much water to apply to replenish the water lost from the soil. Factors that affect determination of the amount of water include the following: (1) type of vegetation; (2) soil type; (3) root depth; (4) topography; (5) micro-climate; and (6) density of vegetation. These factors are explained further below.
Vegetation Type.
For a particular type of vegetation, such as cool season grass, the ET value represents the amount of water that has to be spread over the vegetation to replace the moisture lost by the natural and ongoing process of evaporation and transpiration. Accordingly, ET values are usually normalized to a specific type of plant or crop. For example, various plants require different amounts of moisture in the soil to sustain an optimal appearance and healthy growth environment. Plants which are drought-tolerant require less water than a baseline crop, such as grass, while lush plant types require more water. A crop coefficient (Kc) value is used to adjust the baseline ETo value for a particular plant type. For example, the crop coefficient Kc for shrub-type plants might be 0.5, while the Kc for cool-season grass might be 0.8. In addition, the Kc is also dependent on the time of year. That is, the Kc function is cyclic in nature, with the maximum generally occurring during the spring and the minimum during the winter.
Soil Type.
The ability of soil to absorb and retain applied water is an important consideration in determining how much and how often to water. Sandy soils do not retain water well, so less water with more frequency is needed, or water will percolate beyond the root zone and be wasted. On the other hand, clay soils retain water well, meaning more water with less frequency can be applied. In applying water, the absorption rate also needs to be taken into account to avoid water run off. Sandy soils have a high absorption rate as compared to clay soils. In the latter case, the total amount of water to apply needs to be divided into multiple watering cycles with each cycle having a relatively short watering time with a waiting time between cycles.
Root Depth.
The root zone depth of plants to be watered must also be taken into account. If too much water is applied, the water will percolate beyond the root zone and be wasted. Root zone depth also affects the frequency of watering. A plant with a deep root zone needs less frequent but longer watering times. A plant with a shallow root zone needs more frequent but shorter watering times.
Topography.
Topography is an important consideration in watering, since a steep slope will have a higher amount of run off than a shallow slope. Steeper slopes require multiple cycles with short watering times and wait times between cycles to allow penetration of the applied water into the soil.
Micro-climate.
Micro-climate takes into account existing conditions immediately surrounding the area which is to be watered. These conditions can include fully or partial shaded areas, parking lot areas, park areas with trees, etc. Since shaded areas do not require as much water as sunlit areas, less water is needed. A micro-climate coefficient (Kmc) value is used to adjust the baseline ETo value for a particular site.
Vegetation Density.
Density of the vegetation which is to be watered is also used in determining the amount of water to be applied. As density of vegetation increases, more water will transpire from the leaf area, requiring an increase in the amount of water needed. A vegetation density coefficient (Kd) value is used to adjust the baseline ETo value for a particular plant density.
Although prior art systems have used ET computations to determine watering schedules for irrigation systems, one drawback of such systems has been that they have relied upon current or historical meteorological data. Consequently, an ET value for a particular site or zone can be computed that indicates water should be applied without regard for probable changes in local weather conditions. Thus, a watering schedule may be computed that applies water today without regard for a forecast of precipitation or other significant meteorological events tomorrow.
Other prior systems that use ET data require on-site weather stations or ET measurements, along with central or host computers to perform irrigation watering calculations. These systems are generally not cost-effective for most small landscape sites and can require on-site system operators.
For small landscapes, such as that found at a single home site, prior art practices for water management of irrigation primarily consists of automatic or manual shutoff during rain and seasonal scheduling of use.
Thus, a drawback of conventional irrigation control systems is the lack of capability to account for all the changing weather-related parameters (such as rainfall, temperature, solar radiation, wind speed, humidity, seasonal plant requirements, forecasts, etc.) at a cost-effective price. Use of conventional irrigation control systems results in a deviation from optimum watering schedules and causes either more water to be used or not enough water, which, in turn, stresses the plants.
Accordingly, a need exists for an improved irrigation control system that can cost-effectively schedule watering amounts to optimize use. This can be accomplished by apportioning some functions to the local irrigation site and some to a remote host computer that gathers both predicted and historical meteorological data for the geographic and climatic area where the site is located and that offers an on-line service for ET based scheduling. A further need exists for such an improved system that also uses an ET and/or precipitation calculation that takes into account forecast weather data. The present invention provides a cost-effective method and apparatus that meet these needs.